The present disclosure relates to the field of indoor air ventilation and, more specifically, relates to the regulation of indoor ventilation (e.g., in heating, ventilation, and air conditioning (HVAC) systems).
Ventilation may be regulated for a variety of reasons and in a multiude of ways. For example, the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) has created a set of standards and guidelines relating to ventilation and air quality. These standards and guidelines are often referred to by other promulgators of building codes in order to establish municipal guidelines for safe construction.
ASHRAE Standard 62.2 establishes standards for whole-building ventilation. ASHRAE Standard 62.2-2010 defines a method for determining a continuous ventilation rate that is needed for acceptable indoor air quality. The 2012 and 2015 versions of the International Mechanical Code and International Residential Code (IRC) reference this standard as the basis for the ventilation rates required to meet building code. Numerous other standards bodies cite the ASHRAE standard to define acceptable ventilation rates.
ASHRAE Standard 62.2-2010 allows for intermittent ventilation to meet the continuous ventilation rate requirement. Intermittent ventilation requires that the ventilation flow rate be higher than the required continuous ventilation rate. The ASHRAE standard defines the ventilation flow rate to be Qf=Qr/(ε*), where:
Qf=ventilation flow rate
Qr=continuous ventilation rate
ε=ventilation effectiveness
 =the on time of the vent source divided by the cycle time
Ventilation effectiveness (ε) is a function of the fractional on-time () and the cycle time. Cycle times, as defined in the ASHRAE standard, are bins of 0-4 hours, 8 hours, 12 hours and 24 hours. During the 0-4 hour cycle time bin the effectiveness is equal to one; there is no effectiveness “penalty.” For example, during the 0-4 hour cycle time bin, if the required continuous ventilation rate (Qr) is 75 cubic feet per minute (CFM), an intermittent ventilation flow rate of 150 CFM is allowed as long as the fractional on time is 0.5. This could be accomplished by turning on the 150 CFM flow source for 30 minutes every hour, one hour every two hours, two hours every four hours or any other combination as long as the cycle time is four hours or less and the fraction of the cycle time that the flow source is on is equal to 0.5.
For each successively higher cycle time bin, the effectiveness is reduced as the fractional on-time gets smaller and the ventilation flow rate (Qf) must be increased. For example, if the cycle time bin is selected as 8 hours, and the fractional on-time remains at 0.5, the ASHRAE standard defines the effectiveness to be 0.94 and thus the required ventilation flow rate would be 160 CFM. Alternatively the same 150 CFM flow source could be used but would need to operate at a fractional on time larger than 0.5; the calculation to determine the increased fractional on-time isn't outlined in the body of the ASHRAE standard. Whether resulting in increased airflow or increased fractional on-time, increasing the cycle time has the effect of increasing the total ventilation flow over a 24-hour period and subsequently increasing the energy cost to condition the outside air.
Existing products offer the ability to calculate the fractional on-time based on a known, constant ventilation flow rate and ventilation requirement. Existing products also have the ability to restrict ventilation when the outdoor temperature is too high or too low. If ventilation is restricted in these existing products, however, that time is either lost or added to subsequent a cycle time using an unchanged fractional on-time. Simply disregarding the ventilation on-time that was missed due to temperature or humidity extremes will not meet the ventilation requirement. Adding the missed time without the changing the fractional on-time disregards the effectiveness variable defined in the ASHRAE standard and again will result in the required ventilation not being met.